1. Field of the Invention
An aspect of the present invention may relate to an image displaying apparatus.
2. Description of the Related Art
Recently, an image displaying apparatus is known which may be capable of being placed extremely close to a screen surface (which is a surface to be projected onto of a screen and corresponds to an image surface) compared to conventional ones. Such an image displaying apparatus is called an ultra-short throw projector, etc. Objects of an ultra-short throw projector are, first, to avoid glare caused by projection light entering an eye of a presenter (an expositor or speaker, etc.) standing near a screen, and second, to prevent an audience listening to an explanation of a presenter from being affected by exhaust air or noise of a projector.
For a projection optical system included in an ultra-short throw projector, there are provided a simple extension of an angle of view of a conventional (coaxial or rotationally symmetric) projection optical system to reduce the distance thereof from a screen surface, use of a curved mirror (for example, see Japanese Patent No. 4329863 and Japanese Patent No. 3727543), etc. Herein, it may be possible for a method of extending an angle of view of a projection optical system to achieve an object of ultra-short throw as an extension of a conventional technique. However, it may be necessary for such a method to provide a lens with a large outer diameter near a screen, and hence, a projector may be large entirely. On the other hand, a method of using a curved mirror is compact while it may be possible to provide projection at an ultra-short distance.
An invention disclosed in Japanese Patent No. 4329863 is a method of arranging a concave mirror behind a lens optical system constituting a projection optical system for projection. On the other hand, an invention disclosed in Japanese Patent No. 3727543 is a method of arranging a convex mirror behind a lens optical system for projection. In any of the methods, it may be possible to conduct setting by merely arranging a lens(es) and a mirror(s) in order, and hence, it may be possible to increase the precision of arrangement of parts. However, a long distance between a lens optical system and a mirror may be necessary, and hence, a large projection optical system may be provided.
It is also proposed that it may be possible to reduce a distance between a lens and a mirror (for example, see Japanese Patent Application Publication No. 2009-157223 and Japanese Patent Application Publication No. 2009-145672). Inventions disclosed in Japanese Patent Application Publication No. 2009-157223 and Japanese Patent Application Publication No. 2009-145672 are to arrange a folding mirror to fold a long distance between a lens optical system and a mirror wherein miniaturization of an optical system is intended. Herein, an invention disclosed in Japanese Patent Application Publication No. 2009-157223 is to arrange a concave mirror and a convex mirror in order net to a lens optical system wherein miniaturization is intended. On the other hand, an invention disclosed in Japanese Patent Application Publication No. 2009-145672 is to arrange a plane mirror behind a concave mirror wherein miniaturization is intended.
However, a distance from an image displaying element to a curved mirror is long in an optical system disclosed in any of Japanese Patent Application Publication No. 2009-157223 and Japanese Patent Application Publication No. 2009-145672 (for example, “L1” in FIG. 4 of Japanese Patent Application Publication No. 2009-157223 is long). Accordingly, a length of an optical system body may interfere with reducing a distance from a screen to a projector body even further than conventional ones, and hence, there may be a limitation for an amount of protrusion of a projector body from a screen surface.
It is also proposed that such a constraint for “a size of an optical system itself” be solved (for example, see Japanese Patent No. 4210314). Japanese Patent No. 4210314 discloses a so-called vertical type projection optical system wherein a screen surface and a display surface of an image displaying element are perpendicular to each other. When such a vertical type is adopted, a length of a projection optical system itself may not provide a constraint for an amount of protrusion of a projector body from a screen surface, and hence, an ultra-short throw may be possible.
As result of actively studying miniaturization of such a vertical type projection optical system and an ultra-short throw with a throw ratio of 0.3 or less, it is found that it may be difficult for a configuration illustrated in FIG. 12 of Japanese Patent No. 4210314 to project a large format image even by an ultra-short throw. That is, it is found that it may be necessary to increase a divergence of light incident on a mirror system from a lens optical system or an optical system for imaging an intermediate image between an image displaying element and a screen surface may be preferable in order to project a large format image by an ultra-short throw.
However, as an intermediate image is imaged between an image displaying element and a screen surface, a problem described below may be caused. In an intermediate image method, a Digital Micromirror Device (DMD) as an image displaying element and an intermediate image are conjugate and the intermediate image and a screen surface are conjugate. Hence, as dust, etc., attaches to an optical element arranged near an intermediate image, enlargement thereof on and projection onto a screen surface may occur to provide an image defect. Such an image defect may degrade the quality of a display image.
For a countermeasure against such an image defect, it is also considered that no optical element is to be arranged near an intermediate image. However, such a matter may add a constraint to an optical path layout. That is, when there is a constraint on an optical path layout, it may be difficult to adopt a vertical type, and hence, miniaturization of an image displaying apparatus may be hindered, which may be unfavorable.